1. Field of the Invention
This invention relates to a projection display, and more particularly to a color separation beam splitter for projectors.
2. Description of Prior Art
The color purity of three primary color lights obtained by a conventional tri-color separation beam splitter is not satisfactory since the wavelength range of visible light covers a wide gamut. There are many prior arts trying to solve this problem, such as U.S. Pat. No. 4,507,679 issued to Sidney Bendell, U.S. Pat. No. 5,777,674 of Ohmuro and U.S. Pat. No. 5,777,673 of Yoshikawa, etc. Please refer to FIG. 1a. In Bendell's patent ('679), a space layer 2 is formed between two triangular prisms to serve as a total reflection interface for the second reflected beam. Referring to FIGS. 1b and 1c, Ohmuro and Yoshikawa respectively place air gaps 4a, 4b and 6a, 6b between thin film dichroic mirrors to separate a visible light into a red light, a blue light and two green lights. The four-color separation beam splitter can provide three primary color lights with high color purity. However, such a prism assembly is hard to fabricate and has a large volume since it includes two air gaps and three dichroic coatings.